The use of telecommunications equipment for both personal and enterprise requirements has been dramatically increasing for the past number of years. With this increased use comes the demand for greater efficiency and higher bandwidths. Asynchronous transfer mode (ATM) technology is being seen as the solution of choice to meet these demands. One of the key attributes of ATM technology is its ability to efficiently carry many types of information traffic such as data, voice and video and to provide a variety of qualities of service (QoS) for network applications.
An issue of great importance to users and developers of ATM switches is how to manage the traffic in an efficient manner in order to gain a high degree of bandwidth utilization on a wide area network. The ATM Forum sets out numerous criteria with respect to the nature and requirements of traffic management in ATM systems. Included in these criteria are several traffic descriptors which are basically a set of parameters used to capture the intrinsic characteristics of particular streams of network data. These standard traffic descriptors allow an ATM switch to use statistical models based on the traffic descriptors to efficiently allocate resources (bandwidth, buffers) amongst the various streams of traffic.
Because the allocation of resources in an ATM network is based on some basic parameters that characterize traffic streams, it is important that the traffic streams behave as closely as possible to those traffic descriptor parameters. The more closely the traffic streams adhere to the traffic descriptors, the more accurate the allocation of network resources will be. If the allocation of network resource is very accurate then the usage of these resources will be very efficient while maintaining the required qualities of service for all traffic.
One method of insuring that ATM traffic streams conform to the negotiated traffic contract is to police the data with a usage parameter control (UPC) algorithm. A more desirable method of assuring traffic conformance is to employ traffic shaping. Traffic shaping takes a stream of data and reshapes its attributes so that it conforms to the traffic descriptor. Shaping allows a slightly nonconforming stream to become conforming with no cell loss.
When traffic shaping is employed in a network it is important that the traffic be shaped accurately to conform as closely as possible to the parameters of the traffic contract. A problem can arise when a cell can be inserted into a traffic stream at a point that is downstream from a traffic shaping entity. This cell insertion can cause several problems including violation of the strict traffic descriptor parameters or an instantaneous and unrecoverable increase in cell transfer delay (CTD) for every traffic stream serviced by the shaper. The actual problem encountered depends on the specific details of implementation of the cell insertion and shaping devices.